In a vehicle having an idle reduction function, when engine restart is requested through a driver's driving operation, quick performance of restarting the engine instantly is required. At this time, fuel is injected into a cylinder in a stopped state in an expansion stroke to enable quick restart. It is known that stopping the engine in a specific crank angle range among crank angle ranges in the expansion stroke improves the restarting performance.
In view of this, it is necessary to control the engine stop position to an appropriate position, but the number of revolutions of the engine constantly varies due to pumping of a piston. Further, the rotation reduction behavior (deceleration) differs in each engine due to the friction or the like of the engine. Therefore, there has been a problem in that the engine stop position cannot be accurately controlled to the appropriate position.
As a method for solving such a problem, there have been proposed a method of controlling the engine stop position with use of a power-generation braking torque of a generator (for example, see Patent Literature 1), and a method of controlling the engine stop position with use of a short-circuit braking torque generated by a three-phase short circuit of an armature coil (for example, see Patent Literatures 2 and 3). Note that, details of the controls of Patent Literatures are described later.
Note that, as a type of the generator mounted on the vehicle, there is generally known a generator of a field coil type (synchronous machine type), which is configured to perform a power generation operation by generating an electromotive force through an armature coil with a magnetic flux generated by causing a current to flow through a field coil. Further, there are known two methods for braking the field coil-type generator. One method is power-generation braking using power generation, and the other method is short-circuit braking using a short circuit of the armature coil.
The electromotive force generated in the coil is generally proportional to the speed of the magnetic flux crossing the coil. The generator is rotated in synchronization with the engine via a pulley, and hence the generated voltage of the generator can be increased as the number of revolutions of the engine is increased. Note that, the generator enters a charging state when the generated voltage of the generator is higher than a voltage between terminals of a battery. Therefore, the power generation operation is not carried out when the generated voltage falls below the voltage between the terminals of the battery, and hence no power-generation braking torque is generated.
On the other hand, the torque due to the short-circuit braking is generated by consuming the electromotive force of the armature coil therein. Therefore, the torque is not subjected to the restriction of the battery voltage, and can be generated even in an extremely low rotation range. However, in contrast to a magnet-type generator, which requires no additional power during short-circuit braking because a magnetic flux is constantly generated in a rotator, in the field coil-type generator, it is required to cause a current to flow through the field coil to generate a magnetic flux when the short-circuit braking is carried out. Therefore, excess power is consumed.
Further, because the power-generation braking torque is dependent on the battery voltage and the short-circuit braking torque is independent of the battery voltage, it is understood that braking torque characteristics during a power-generation braking mode and braking torque characteristics during a short-circuit braking mode do not generally match with each other.
Note that, in Patent Literature 1, there is disclosed a method of controlling the stop position with use of the power-generation braking torque. In this method, as described above, a sufficient power-generation braking torque cannot be generated in a low rotation range. Therefore, the power-generation braking torque is controlled so as to be matched with a target number of revolutions for the rotation stop (rotation reduction behavior). With this, the behavior of the number of revolutions in the low rotation range, which is uncontrollable by the power-generation braking torque, is set uniform, thereby stopping the engine in a specific crank angle range.
Further, in Patent Literature 2, there is disclosed a method of controlling the stop position with use of the short-circuit braking torque. In this method, as described above, the short-circuit braking torque is generated also in the low rotation range in which the power-generation braking torque cannot be generated, thereby stopping the engine accurately in the vicinity of the target stop position.
Further, in Patent Literature 3, there is disclosed a method of controlling the stop position with use of the short-circuit braking torque similarly to Patent Literature 2. In this method, when the number of revolutions of the engine is less than a predetermined number of revolutions, an energization phase of a motor is short-circuited to generate a short-circuit braking torque, thereby stopping the engine.